Lignocellulose biomass in the natural world, such as wood and grasses, is roughly composed of 50% cellulose, 25% hemicellulose and 20% lignin. Cellulose is mainly used for papermaking. Meanwhile, cellulose and hemicellulose can be degraded by hydrolysis to saccharides which are then used for producing ethanol. The existing methods for dissolving lignocellulose biomass mainly use a batch or semi-flow percolating reactor to carry out the dissolution reaction at a temperature of 200˜300° C. for 15 minutes. Only approximately 40˜60% of wood or grasses can be dissolved and hydrolyzed to hydrosoluble substances by using such methods. Thus, the methods are not only very low in production efficiency, but also too long in reaction time, which easily results in secondary decomposition of the saccharide products. The latest researches show that high-pressure hot water is a weakly polar solvent exhibiting both acidic and basic properties, and therefore can dissolve biomass and enables the hydrolysis reaction to occur in homogeneous phase. Sasaki et al. found that cellulose can be completely dissolved in water at 320° C. and at a water density of more than 1000 kg/m3 (Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T. & Arai, K. “Dissolution and hydrolysis of cellulose in subcritical and supercritical water”, Ind. Eng. Chem. Res. 39, 2883-2890, 2000). Later, Ogihara et al. further found that, over a range of water densities of 550˜1000 kg/m3, for the temperatures at which cellulose is completely dissolved, there is a minimum of 320° C. at water density of 850 kg/m3 (Ogihara Y.; Smith Jr., R. L.; Inomata H. & Kunio A., “Direct observation of cellulose dissolution in subcritical and supercritical water over a wide range of water densities (550˜1000 kg/m3), Cellulose, 12, 595-606, 2005). However, the above findings are all directed to pure cellulose, and thus are hard to be applied in industry because natural wood or grasses only contain about 50% cellulose and the process for separating cellulose is complicated and costly. Therefore, the technical problem that needs to be solved urgently in the art is how to realize the dissolution and rapid hydrolysis of natural lignocellulose biomass. On this basis, the present inventor found by study that the addition of a alkaline solution in pure water enables lignocellulose biomass such as wood flour to be completely dissolved and homogeneously hydrolyzed at 329° C.˜367° C. A Chinese Patent Application was then filed accordingly (No. 200710141265.3). The addition of an acidic or an alkaline solution also enables cellulose to be completely dissolved and homogeneously hydrolyzed at a relatively low temperature (for example, 261˜352° C.) (No. 201010104133.5). In the above methods, however, after adding an acidic or an alkaline catalyst solution, the production cost will be increased, the catalyst will not be recovered so easily and the environment will be polluted easily.